1. Field of the Invention
The present invention relates to an inspection method of radiation imaging system for inspecting quality of a radiation imaging system which includes an image reading apparatus for reading image information from a sheet recorded with a radiation image, and relates to a medical image processing apparatus using such inspection method. Further, the present invention relates to a phantom to be used for image quality control of a radiation image during the inspection of such radiation imaging system.
In this application, the word “radiation” is used in a wide sense so as to include a corpuscular beam such as an electron beam and an electromagnetic wave in addition to general radiation including X-ray, α-ray, β-ray, γ-ray, ultraviolet ray and so on.
2. Description of a Related Art
Conventionally, an imaging method using a radiation (X-ray, α-ray, β-ray, γ-ray, electron beam, ultraviolet ray and so on) is utilized in various fields, and particularly, employed as one of the most important means for diagnosis in a medical field. Since a first X-RAY photograph was realized, X-ray photography has been repeatedly improved and a method using a combination of a fluorescent screen and an X-ray film is predominantly used at present. On the other hand, in recent years, various digitized apparatuses such as X-ray CT apparatus, ultrasonic imaging apparatus, or MRI apparatus are practical use and construction of a diagnosis information processing system in hospitals is being developed. As for X-ray images, many studies have also been made for digitizing the systems, and a radiation imaging method using photostimulable phosphor has been established, whereupon there has been an increasing interest in such techniques available as methods that will replace conventional X-ray photography.
The photostimulable phosphor (storage phosphor) is a substance which accumulates a part of radiation energy when irradiated with a radiation; and after that, emits photostimulable luminescent light corresponding to the accumulated energy when irradiated with an excitation light such as visible light. The presence of that has been long known. The radiation imaging method using the photostimulable phosphor will be described below. First, using an imaging apparatus, a radiation image of an object such as human body is taken on a sheet, to which photostimulable phosphor is applied, and recorded thereon. Then, using an image reading apparatus, the photostimulable phosphor sheet is scanned with excitation light such as a laser beam, and thereby photostimulable luminescent lighe is read out photoelectrically by a photo-multiplier of the image reading apparatus. Based on the analog image signals obtained as described above, digital image data is obtained. Further, after being appropriately processed by using a medical image processing apparatus, the image data is outputted to a display such as a CRT, or printed out on a film with a laser printer or the like. Consequently, a radiation image, in which the energy level of the radiation transmitted through the object is visualized by means of gray levels or gradation, is obtained.
When such radiation image is utilized for medical diagnosis, high reliability in the radiation imaging system has to be highly ensured. Therefore, the performance of the radiation imaging system has to be measured and verified as needed. The reason for this is that, when any portion of radiation imaging system including photostimulable phosphor sheet and image reading apparatus is degraded or any abnormality occurs thereon, no normal radiation image can be obtained, and the reliability in image analysis is reduced. Further, since the irradiation of high-level radiation energy adversely affects human body, the verification of the performance of the radiation imaging system is also important in view of safety.
Japanese Unexamined Patent Application Publication JP-2000-275758A (pp. 6-8, FIG. 1) discloses a radiation image reading apparatus in which the photoelectric reading means is prevented from being adversely affected even when the open/close member of the housing is opened, maintenance person is prevented from being accidentally exposed to the excitation light exceeding a prescribed level, and further, tests such as verification of sheet conveyance situation and so on can be carried out in the situation where the open/close member is opened.
In the above-mentioned radiation image reading apparatus disclosed in JP-2000-275758A, test image signals are previously prepared, and based on the test image signals, image data is generated. However, in this radiation image reading apparatus, no test can be carried out in the processes from the generation of the photostimulable luminescent light to the generation of the image data based on the image signal which is represented by means of photostimulable luminescent light.
Also, in the following patent documents, inspection methods of easily detecting abnormality of a radiation image filming apparatus are disclosed. In the inspection method disclosed in Japanese Unexamined Patent Application Publication JP-2002-277992A (pp. 4-5, FIG. 3), ultraviolet ray is uniformly irradiated over the entire surface of a photostimulable phosphor detector (sheet), and photostimulable phosphor, which is generated by irradiating an excitation light on the photostimulable phosphor detector, is photoelectrically amplified. However, according to this inspection method, only limited items such as S-value representing the density of output image, uniformity of the density, granularity and the like can be inspected.
Japanese Unexamined Patent Application Publication JP-2002-277993A (PP. 3-4, FIG. 3) discloses an inspection method in which no additional imaging for the purpose of inspection is needed because image data obtained by imaging an object is used for the inspection. Further, Japanese Unexamined Patent Application Publication 2002-277995 (pp. 3-4, FIG. 4) discloses an inspection method in which excitation light, which is modulated such that the space of non-radiated area changes wider and narrower, is irradiated on a photostimulable phosphor detector (sheet), and then, difference between the read out signal (image signal) from the photostimulable phosphor detector and the modulation state of the excitation light is numerically calculated. However, according to these inspection methods, only limited items such as S-value, resolution and the like can be inspected.
In an inspection method disclosed in Japanese Unexamined Patent Application Publication JP-2002-278004A (pp. 4-5, FIG. 3), test light of blue light irradiated from a light source disposed in a position adjacent to a photostimulable phosphor detector (sheet) is guided to a photo-multiplier by using a light collection unit and amplified photoelectrically by using a photo-multiplier. However, according to this inspection method, only limited items such as light collection unit and the like can be inspected.
Accordingly, a method is desired which is capable of effectively inspecting radiation imaging system including radiation image reading apparatus.
Meanwhile, when inspecting a radiation imaging system, a physical phantom or a body imitation phantom is used. For example, in Japanese Unexamined Patent Application Publication JP-A-11-4822, there are disclosed image quality test phantom and method of automatic monitoring and evaluation of image quality in a digital X-ray visualization and imaging system. The physical phantom, which is also referred to as QC (quality control) phantom, is a phantom in which various members made by using materials such as metal and resin of which radiation absorption coefficients are known, are disposed on a base plate. Each of the members has a predetermined size, shape, density, composition and so on, and they are used as image quality evaluating patterns. Those image quality-evaluating patterns are designed so as to enable measurement of one or plural image quality evaluation items pertaining to a radiation imaging system.
The radiation imaging is performed by irradiating a radiation such as X-ray on a QC phantom as described above, and the radiation image information of the QC phantom is recorded on a recording medium such as the photostimulable phosphor sheet. The recording medium is subjected to a predetermined processing to generate a radiation image, which is displayed on a CRT monitor or the like. This radiation image is analyzed as to predetermined image evaluation items so that the constancy or invariance of various performance parameters of the radiation imaging system are evaluated thereby the quality inspection of the radiation imaging system is carried out.
The evaluation of the constancy or invariance of the performance parameters is made in a manner of quantitative evaluation and visual evaluation. The quantitative evaluation means an inspection in which the evaluation is made quantitatively by processing including calculation processing of digital data. On the other hand, the visual evaluation means an inspection to be performed in a manner other than the above quantitative evaluation, for example, an inspection in which an operator evaluates a radiation image in a manner of visual observation. In these inspections, the visual evaluation can be easily carried out relatively. However, since the visual evaluation depends on the operator's subjective feeling, no objectivity can be expected. On the other hand, the quantitative evaluation can provide objective evaluation but requires a lot of labor for the operation. Generally, the visual evaluation is carried out more frequently than the quantitative evaluation. For example, the visual evaluation is carried out every week, while the quantitative evaluation is carried out once per three months. Further, in the case where the constancy evaluation is made in large-scale facilities, ten or more reading apparatus are usually provided. In view of such circumstances, there is a need for an efficient workability.
However, an ordinary QC phantom is not designed to enable both of the visual evaluation and the quantitative evaluation for one image quality evaluation item. For example, in the case where objective measurement results of image quality parameters are required after a visual evaluation, it is therefore required to mount a new phantom specialized or dedicated for quantitative evaluation and resume the measurement starting from the radiation imaging. That is, it is necessary to change one to another of dedicated phantoms depending on which of the visual evaluation and the quantitative evaluation is made. Accordingly, the verification operation of the radiation imaging system becomes complicated, and therefore, a lot of labor as well as time consumption are required.
Further, when the visual evaluation and the quantitative evaluation are carried out by using different phantoms, there arises a difficulty to compare the measurement results thereof and consider the measurement results. Accordingly, result of the visual evaluation cannot be evaluated from further objective viewpoint. Therefore, no precise constancy evaluation can be obtained, and there arises such problem that no reliability in the evaluation can be ensured. Furthermore, since two phantoms for visual evaluation and quantitative evaluation are required, there occurs another problem that the cost thereof increases, that is, uneconomical.
Still further, in order to ensure the reliability in the inspection accuracy, it is required that an image of QC phantom is displayed at a proper reference region in the radiation image by mounting the QC phantom in a proper position corresponding to a reference position or an inspection object area within the recording medium at the time of imaging. Therefore, in the case where the QC phantom is mounted out of the proper position, the image of the QC phantom is not display ed in the proper reference region in the radiation image, and the imaging has to be carried out again after correcting the position of the QC phantom. This may lead to the interruption as well as the complication of the inspection operation. Thus, there arises still another problem that automation of the inspection operation is largely prevented.